Conventionally, as a method for cleaning soils and groundwater contaminated with organohalogen compounds, such as trichloroethylene, there has been proposed a treatment method using iron powder having an increased specific surface area as a decomposer to make it possible to change the contaminated soils and groundwater to be harmless at a relatively low cost and in a relatively short term (refer to Patent document 1). Further, there has been developed and used a technique of using iron powder containing copper as a decomposer for improving the performance for decomposing an organohalogen compound (refer to Patent document 2).